U.S. patent application number 10/489049 was filed with the patent office on 2004-12-02 for electronic device and composition.
Invention is credited to Balkenende, Abraham Rudolf, De Theije, Femke Karina, Kriege, Jan Cornelis.
Application Number | 20040238901 10/489049 |
Document ID | / |
Family ID | 26076994 |
Filed Date | 2004-12-02 |
United States Patent
Application |
20040238901 |
Kind Code |
A1 |
Balkenende, Abraham Rudolf ;
et al. |
December 2, 2004 |
Electronic device and composition
Abstract
The electronic device with a layer of mesoporous silica can be
obtained by applying a composition comprising alkoxysilane, a
surfactant and a solvent onto a substrate, and by subsequently
removing the surfactant and the solvent. The customary
dehydroxylation treatment is not necessary if the composition
contains a mixture of tetra-alkoxysilane, particularly
teatraethoxyorthosilicate (TEOS), and an alkyl-substituted
alkoxysilane, particularly a phenyl-substituted, methyl-substituted
or ethyl-substituted trialkoxysilane. If both silanes are present
in a molar ratio of approximately 1:1, a layer with a dielectric
constant of 2.5 or less is obtained.
Inventors: |
Balkenende, Abraham Rudolf;
(Eindhoven, NL) ; De Theije, Femke Karina;
(Eindhoven, NL) ; Kriege, Jan Cornelis;
(Eindhoven, NL) |
Correspondence
Address: |
PHILIPS ELECTRONICS NORTH AMERICA CORPORATION
INTELLECTUAL PROPERTY & STANDARDS
1109 MCKAY DRIVE, M/S-41SJ
SAN JOSE
CA
95131
US
|
Family ID: |
26076994 |
Appl. No.: |
10/489049 |
Filed: |
March 8, 2004 |
PCT Filed: |
September 12, 2002 |
PCT NO: |
PCT/IB02/03787 |
Current U.S.
Class: |
257/406 ;
257/E21.26; 257/E21.273 |
Current CPC
Class: |
C09D 183/04 20130101;
C09D 183/02 20130101; H01L 21/3121 20130101; H01L 21/02126
20130101; H01L 21/02216 20130101; H01L 21/02203 20130101; H01L
21/02337 20130101; H01L 21/02282 20130101; C01B 37/02 20130101;
H01L 21/31695 20130101 |
Class at
Publication: |
257/406 |
International
Class: |
H01L 029/76 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 17, 2001 |
EP |
01203536.6 |
Mar 28, 2002 |
EP |
02076262.1 |
Claims
1. An electronic device comprising: a substrate provided on one
side with a mesoporous layer containing silica, which layer can be
obtained by applying a liquid layer of a composition comprising
tetra-alkoxysilane, aryl-substituted or alkyl-substituted
alkoxysilane, a surfactant and a solvent onto a substrate, wherein
the molar ratio between the tetra-alkoxysilane and the
aryl-substituted or alkyl-substituted alkoxysilane is 3:1 at the
most; and by removing the surfactant and the solvent from the
liquid layer, thereby forming the hydrophobic, mesoporous
layer.
2. An electronic device as recited in claim 1, wherein a first and
a second conductor are present which are electrically insulated
from each other by the mesoporous layer; and the mesoporous layer
has a relative dielectric constant below 3.0.
3. An electronic device as recited in claim 2, wherein the
mesoporous layer has a porosity above 45%.
4. An electronic device as recited in claim 1, wherein the
aryl-substituted or alkyl-substituted alkoxysilane is selected
among the group formed by C.sub.1-C.sub.3-alkyl and
phenyltrialkoxysilanes and fluoridized analogues thereof, which
alkoxy group is selected among the group formed by methoxy, ethoxy,
propoxy and butoxy.
5. An electronic device as recited in claim 4, wherein the
aryl-substituted or alkyl-substituted alkoxysilane is
methyltrimethoxysilane (MTMS).
6. A composition comprising: tetra-alkoxysilane, aryl-substituted
or alkyl-substituted alkoxysilane, a surfactant and a solvent,
wherein the molar ratio between the tetra-alkoxysilane and the
aryl-substituted or alkyl-substituted alkoxysilane is below
3:2.
7. A composition as recited in claim 6, wherein the weight ratio of
the surfactant to the total amount of alkoxysilanes is in excess of
0.15:1.
8. A composition as recited in claim 6, wherein the
aryl-substituted or alkyl-substituted alkoxysilane is selected
among the group consisting of
C.sub.1-C.sub.3-alkyltrialkoxysilanes, which alkoxy group is
selected among the group consisting of methoxy, ethoxy, propoxy and
butoxy.
9. A method of preparing a mesoporous layer comprising: the
provision of a liquid layer of a composition comprising
tetra-alkoxysilane, aryl-substituted or alkyl-substituted
alkoxysilane, a surfactant and a solvent onto a substrate, wherein
the molar ratio between the tetra-alkoxysilane and the
aryl-substituted or alkyl-substituted alkoxysilane is 3:1 at most;
and removing the surfactant and the solvent from the liquid layer,
thereby forming the hydrophobic mesoporous layer.
10. A method as recited in claim 9, wherein the composition that is
applied comprises: tetra-alkoxysilane, aryl-substituted or
alkyl-substituted alkoxysilane, a surfactant and a solvent, wherein
the molar ratio between the tetra-alkoxysilane and the
aryl-substituted or alkyl-substituted alkoxysilane is below
3:2.
11. The method of claim 10 wherein the composition that is applied
further comprises, the weight ratio of the surfactant to the total
amount of alkoxysilanes is in excess of 0.15:1.
12. The method of claim 10 wherein the composition that is applied
further comprises, the aryl-substituted or alkyl-substituted
alkoxysilane is selected among the group consisting of
C.sub.1-C.sub.3-alkyltrialkoxysila- nes, which alkoxy group is
selected among the group consisting of methoxy, ethoxy, propoxy and
butoxy.
Description
[0001] The invention relates to an electronic device comprising a
substrate provided on one side with a mesoporous layer containing
silica, which can be obtained by, inter alia, applying a layer of a
composition comprising a tetra-alkoxysilane, an alkyl-substituted
alkoxysilane, a surfactant and a solvent, and by removing the
solvent and the surfactant, thereby forming the mesoporous
layer.
[0002] The invention also relates to a composition comprising
tetra-alkoxysilane, aryl-substituted or alkyl-substituted
alkoxysilane and a solvent.
[0003] The invention further relates to a method of preparing a
mesoporous layer comprising the application of a liquid layer of a
composition containing tetra-alkoxysilane, aryl-substituted or
alkyl-substituted alkoxysilane, a surfactant and a solvent onto a
substrate, and removing the surfactant and the solvent from the
liquid layer, thereby forming the hydrophobic, mesoporous
layer.
[0004] Such an electronic device is known from WO-A 00/39028.
Example 5 discloses a composition comprising
tetraethoxyorthosilicate and methyltriethoxysilane. Said
tetraethoxyorthosilicate, also referred to as TEOS, is a frequently
used tetra-alkoxysilane. Tetra-alkoxysilanes will hereinafter also
be referred to as TEOS. Methyltriethoxysilane, also referred to as
MTES, is an example of an aryl-substituted or alkyl-substituted
alkoxysilane. A further example thereof is methyltrimethoxysilane,
also referred to as MTMS. Such aryl-substituted or
alkyl-substituted alkoxysilanes will hereinafter also be referred
to as ASAS.
[0005] The known composition comprises TEOS and MTES in a ratio of
0.85:0.15. For the surfactant use is made of 10 lauryl ether, also
referred as C.sub.12H.sub.25(CH.sub.2CH.sub.2O).sub.10OH. The
solvent is a 50/50 mixture of water and ethanol. Furthermore,
hydrogen chloride is used as the catalyst. The
surfactant:silane:water:ethanol:hydrogen chloride ratio is
0.17:1:5:5:0.05. After ageing for 20 hours, this composition is
applied to silicon slices by means of spin coating at 2000 rpm for
30 seconds. The solvent and the acid are removed in 1 hour at 115
.degree. C., after which the surfactant is completely removed by
calcination at 475 .degree. C. for 5 hours. Finally, a
dehydroxylation, process takes place by exposing the mesoporous
layer to a silane, such as a 10% solution of hexamethyldisilazane
in toluene, and subsequently to a vacuum treatment, which
dehydroxylation process is repeated a number of times at
temperatures in the range between 25 and 450 .degree. C. The
resulting layer may be present in a semiconductor device, in
particular as a dielectric between two conductors in an
interconnect structure, on account of the low dielectric constant.
The relative dielectric constant, in relation to the dielectric
constant of a vacuum, is 2.25.
[0006] A drawback of the known electronic device resides in that a
dehydroxylation aftertreatment is required. Said aftertreatment
renders the mesoporous layer hydrophobic, however, it is by no
means a certainty that the layer becomes completely hydrophobic.
Moreover, it is possible that subsequent steps in the manufacturing
process annihilate the results of the aftertreatment. Besides, said
aftertreatment involves at least one additional step in the
manufacturing process.
[0007] Therefore, it is a first object of the invention to provide
an electronic device of the type mentioned in the opening
paragraph, by means of which a stable, mesoporous layer can be
obtained without a dehydroxylation aftertreatment.
[0008] The first object is achieved in that TEOS and ASAS are
present in a molar ratio of 3:1 at the most.
[0009] By using a composition comprising a mixture of TEOS and one
or more aryl-substituted or alkyl-substituted alkoxysilanes, a
stable layer is obtained that does not require a dehydroxylation
aftertreatment. The invention is based on the recognition that the
formation of a silica network from the alkoxysilanes requires less
than four alkoxy groups per silicon atom. Any remaining alkoxy
groups and the silanol groups formed after hydrolysis render the
silica network hydrophilic. In relation to TEOS, ASAS contains
fewer alkoxy groups. On the other hand, ASAS comprises more
hydrophobic aryl or alkyl groups. These alkyl groups have a
hydrophobic, apolar character and preclude water adsorption in the
porous silica network.
[0010] The solvent and the surfactant are preferably removed in a
treatment at an increased temperature. The increased temperature is
in the range of about 150 to 500 .degree. C. The treatment wherein
solvent and surfactant are removed and a polysilicate coating is
formed, is per se known as sol-gel processing.
[0011] The hydrophobic character of the mesoporous layer in the
device in accordance with the invention implies that essentially no
water adsorption takes place up to an air humidity degree of
approximately 50%. This is sufficient in actual practice since the
air humidity degree in clean rooms can be maintained between 40 and
50%. The device may be exposed to a higher degree of air humidity
during operation, however, an electronic device is customarily
encapsulated in a layer to protect it against moisture. With a
decreasing ratio of tetra-alkoxysilane to aryl-substituted or
alkyl-substituted alkoxysilane the sensitivity to air humidity
decreases until the layers are completely insensitive to air
humidity. It has been found that layers that can be obtained using
a composition comprising TEOS and ASAS in a molar ratio above 3:2
are insensitive to air humidity. Preferably the molar ratio is
below 1:3, which provides an excellent mechanical stability.
[0012] Although it is stated in the prior art that compositions
have been prepared wherein the molar ratio between TEOS and ASAS is
below 5:1, the prior art does not comprise measuring results to
substantiate this. Besides, a dehydroxylation step has been carried
out. The conclusion drawn from that is that the result obtained by
means of the invention was not achieved in the prior art.
[0013] From the article "Synthesis of ordered mesoporous
organic-inorganic thin films" by Balkenende et al, Book of
Abstracts, Conference on nanostructured materials made from
self-assembled molecules and particles, Hindas (Sweden), 2001, a
composition is known with a molar ratio between tetra-alkoxysilane
and methyltriethoxysilane of 1:3 and 1:1. The layers formed are
subjected to an aftertreatment at a temperature in the range from
350 to 800 .degree. C. However, for persons skilled in the art
there is no reason to believe that, without a dehydroxylation
aftertreatment, a mesoporous layer can be obtained exhibiting
hardly any water adsorption at air humidity degrees up to 50% or
higher.
[0014] In a first embodiment of the device in accordance with the
invention, the mesoporous layer is a transmission layer. Said
transmission layer may be part of an interference filter. The
stability up to high humidity levels and the low refractive index
enable a desired filtering characteristic to be efficiently
realized. The transmission layer can also be used in display
devices, such as at the surface of CCDs and LCDs, and in
field-emission displays. For this reason, it is desirable for the
molar ratio between TEOS and ASAS to be below 3:2. At said ratio, a
mesoporous layer having a very low refractive index is obtained,
which is not dependent on the air humidity. Using MTMS as the ASAS,
at said molar ratios and porosity levels above 50%, refractive
indices between 1.15 and 1.22 are obtained.
[0015] In a second embodiment of the device in accordance with the
invention, a first and a second conductor are present which are
electrically insulated from each other by the mesoporous layer
having, in this embodiment, a relative dielectric constant below
3.0. An example thereof is a semiconductor device comprising the
mesoporous layer as an intermetallic or intrametallic dielectric.
The conductors may be present in different layers on the substrate.
It is alternatively possible for the conductors to be situated in
the same layer where they are laterally separated from each other.
Another example is a network of passive components. Such a network
is known from, for example, PCT-application WO-A 01/61847. In this
case, the mesoporous layer is applied to separate a first and a
second winding of a coil from each other. Such a network can of
course also be integrated in an interconnect structure of a
semiconductor device. The device may alternatively be a
bulk-acoustic wave resonator. Such a device is known from patent
application EP-A-1067685. Furthermore, the mesoporous layer may be
situated directly on the substrate or in the substrate so as to
form a buried oxide. In this manner, electrical losses to the
substrate can be reduced substantially. The applications WO-A
01/61847 and EP-A-1067685 are incorporated in this application by
reference.
[0016] A first advantage of the electronic device in accordance
with the invention resides in that a layer is obtained having a
uniform pore size below 10 nm. By virtue of said pore size, the
layer can suitably be used in an integrated circuit having a very
high resolution up to, for example, 70 or 100 nm. If at least part
of the pores would be larger than several nanometers, a barrier
layer of, for example, TaN to be applied to the mesoporous layer
can no longer be provided so as to cover the entire mesoporous
layer. As a result of the fact that this barrier layer is not
tight, impurities in the form of Cu ions (in the case of Cu
metallization) can disturb the properties of the layer or the
device. If the size of the pores is of the order of the distance
between the metal lines, short-circuits may occur between a first
and a second conductor on either side of the mesoporous layer.
[0017] It is particularly preferred to provide a mesoporous layer
with pore sizes below 8 or even below 5 nm. Such layers can be for
instance obtained with the use of a surfactant as
cetyltrimethylammoniumbromide (CTAB). On such a mesoporous layer a
barrier layer with a thickness below 10 nm can be applied with
success, for instance with Atomic Layer Chemical Vapour Deposition
(ALCVD). The resulting stack of mesoporous layer and barrier layer,
wherein the mesoporous layer is etched according to desired
pattern, is suitable for damascene processing, as known per se to
the skilled person.
[0018] A second advantage of the electronic device in accordance
with the invention resides in that the mechanical properties of the
mesoporous layer are better than those of other types of known
mesoporous layers. From S. Yang et al., Chem. Mater. 14(2002),
369-374, for example, a mesoporous layer of
poly(methylsilsesquioxane) or MSQ is known having porosities
ranging from 30 to 50% and hardness levels of 0.28 GPa at a
porosity of 40% and 0.16 GPa at a porosity of 50%. The mesoporous
layer in accordance with the invention, however, enables hardness
levels of 0.6-0.8 GPa to be obtained at porosity levels between 40
and 45%, and a hardness of 0.5 Pa at porosity levels between 52 and
60%.
[0019] In a favorable embodiment of the electronic device in
accordance with the invention, the mesoporous layer has a porosity
above 45%. These higher porosity levels are obtained by increasing
the surfactant content in the composition. It has surprisingly been
found that the stability of the mesoporous layer in accordance with
the invention remains good at higher surfactant contents. In the
method in accordance with the prior art, however, a larger amount
of surfactant causes the layer formed to become unstable after
calcination. Said unstability means that the network of porous
silica collapses, causing the porosity to decrease substantially
from 55 to 28%. The advantage of a higher porosity is, in
particular, that a lower dielectric constant is obtained. A
relative dielectric constant of 1.7 has been achieved.
[0020] Favorable effects are achieved by using an alkyl- or
aryl-substituted alkoxysilane wherein the alkyl respectively aryl
group is selected among a methyl group, an ethyl group and a phenyl
group, or wherein the alkyl group is fluoridized. Such
phenyl-substituted, methyl-substituted and ethyl-substituted
alkoxysilanes are thermally stable up to approximately 450.degree.
C., allowing them to be calcined in the customary manner.
Preferably the alkoxy group is a butoxy, propoxy, ethoxy or methoxy
group. Said thermal stability is particularly favorable for
semiconductor devices which are subjected to a heating step at
approximately 400.degree. C. before the encapsulation is
provided.
[0021] The alkyl- or aryl-substituted alkoxysilane may additionally
be a trialkylalkoxysilane, a dialkyldialkoxysilane and an
alkyltrialkoxysilane or aryl-substituted analogues. Particularly
favorable examples are methyltrimethoxysilane,
methyltriethoxysilane, phenyltrimethoxysilane,
phenyltriethoxysilane. What is meant here is that, by virtue of the
crosslinking of the three alkoxy groups, such alkyltrialkoxysilanes
can be integrated very readily in the silica network, and that, for
this reason, the stability of the network decreases hardly, if at
all, in relation to a network of pure TEOS.
[0022] Particularly favorable results are obtained by using a
composition comprising TEOS and an ASAS, in particular MTMS, in a
molar ratio below 3:2. By using this composition, a mesoporous
layer can be obtained having a low dielectric constant
(68.sub.r<2.6) and a high stability, even in humid conditions.
Measurements have shown that at varying degrees of humidity,
including relative humidity levels in excess of 80%, the refractive
index changes hardly, if at all. This means, inter alia, that a
mesoporous layer can be obtained whose porosity is higher than that
of a mesoporous layer of pure TEOS. As will be understood by
persons skilled in the art, a low dielectric constant is very
important in the manufacture of transistors having comparatively
small channel lengths. Said reduction of the channel length to 100
nm or less causes the RC delay to become one of the factors that
determine the addressing speed of transistors. At the same time,
the resistance increases owing to the smaller width of metal
tracks. Also the capacitance tends to increase owing to the smaller
distance between metal lines. As a result, the use of layers whose
dielectric constant is lower than that of SiO.sub.2 (68.sub.r=4.2)
is necessary.
[0023] For the surfactant use can be made of cationic, anionic and
non-ionic surfactants. Examples are, inter alia,
cetyltrimethylammoniumbr- omide and cetyltrimethylammoniumchloride,
triblock copolymers of polyethylene oxide, polypropylene oxide,
polyethylene oxide ethers, such as polyoxyethylene (10) stearyl
ether.
[0024] Favorable results are achieved using a cationic surfactant
in combination with a molar ratio between said surfactant and the
totality of alkoxysilanes in excess of 0.1:1. In this manner,
layers can be achieved having a relative dielectric constant below
2.5. Unlike mesoporous layers prepared from pure TEOS, the
mesoporous layers manufactured as described above remain stable,
even if the composition comprises a high surfactant content. The
resultant layers have a porosity above 50% and were found to be of
good quality. Although heating is by no means necessary, it can be
carried out under reducing conditions, for example in an atmosphere
of nitrogen and hydrogen. It has been found, as is shown in Table
2, that heating in these reducing conditions at increasing
porosities results in a reduction of the dielectric constant.
[0025] Favorable results have also been achieved using a triblock
copolymer comprising polyethylene oxide, polypropylene oxide and
polyethylene oxide as the blocks serving as the surfactant. An
example of such a surfactant is known by the name of Pluronic F127.
Low concentrations of this surfactant in the composition already
lead to a mesoporous layer having a high porosity and a low
dielectric constant.
[0026] A composition comprising a TEOS, an ASAS, an ionic
surfactant and a solvent is known from Balkenende et. al., Book of
Abstracts, Conference on nanostructured materials made from
self-assembled molecules and particles, Hindas (Sweden), 2001. In
the known composition, the alkyl or aryl-substituted alkoxysilane
is phenyltriethoxysilane (PhTES). The ionic surfactant is
cetyltrimethylammoniumbromide and the solvent is a 80/20 mixture of
ethanol and water that has been acidified. The molar ratio between
TEOS and PhTES is 3:1. The molar ratio between the surfactant and
the totality of alkoxysilane, i.e. TEOS+PhTES, is 0.1:1. The
composition is applied to a substrate and heated to 350.degree. C.
This results in a mesoporous layer having a porosity of
approximately 45%.
[0027] A second object of the invention is to provide a composition
enabling a mesoporous layer to be manufactured having a relative
dielectric constant below 2.6, which dielectric constant is
essentially insensitive to the degree of air humidity.
[0028] It is a third object of the invention to provide a method of
the type mentioned in the opening paragraph, by means of which a
mesoporous layer having a relative dielectric constant below 2.6
can be obtained, which dielectric constant is essentially
insensitive to the degree of air humidity.
[0029] Said second object is achieved in that the molar ratio
between the tetra-alkoxysilane and the aryl-substituted or
alkyl-substituted alkoxysilane is below 3:2.
[0030] It has been found that the composition in accordance with
the invention enables a layer having the desired properties to be
obtained. In addition, the compositions in accordance with the
invention can be used to manufacture mesoporous layers having a
higher porosity. The layers obtained have the advantage, as
compared to the layers known from WO-A-00/39028, that they are
stable without a dehydroxylation aftertreatment. In particular, it
has also been found that the layers formed by means of the
composition in accordance with the invention have a good mechanical
stability, which could not be expected on the basis of the known
composition.
[0031] In all cases it applies that such a composition, in which
the molar ratio between TEOS and ASAS is above 3:1, does not have a
low and stable dielectric constant. In a particularly favorable
embodiment, the ASAS used is methyltrimethoxysilane.
[0032] For the surfactant use can be made of cationic, anionic and
non-ionic surfactants. Examples are, inter alia,
cetyltrimethylammoniumbr- omide (CTAB) and
cetyltrimethylammoniumchloride, triblock copolymers of polyethylene
oxide, polypropylene oxide, polyethylene oxide ethers, such as
polyoxyethylene (10) stearyl ether. Preferably, the surfactant is
present in concentrations above 0.15 g per gram of alkoxysilane. In
the case of a surfactant like CTAB, this means that the
concentration is in excess of 0.1 mol per mol of alkoxysilane. This
leads to a substantial increase in porosity and reduction of the
dielectric constant. Nevertheless, the mechanical stability is
surprisingly good.
[0033] The third object is achieved in that the molar ratio between
TEOS and ASAS is 3:1 at the most. In a favorable embodiment of the
method in accordance with the invention, the composition in
accordance with the invention is used. Preferably, the removal of
the solvent and the surfactant, while forming the mesoporous layer,
takes place by first drying the liquid layer and subsequently
heating it to a temperature in the range from 350 to 450.degree.
C.
[0034] These and other aspects of the electronic device, the
composition and the method in accordance with the invention will be
explained in greater detail with reference to a drawing and Tables,
in which:
[0035] FIG. 1 is a diagrammatic, cross-sectional view of the
electronic device;
[0036] FIG. 2 shows the influence of the surfactant concentration
in the composition on the porosity of the layer obtained;
[0037] FIG. 3 shows a relation between the dielectric constant and
the porosity;
[0038] FIG. 4 shows the influence of the degree of humidity of the
environment on the refractive index of mesoporous layers formed in
accordance with known methods;
[0039] FIG. 5 shows the influence of the degree of humidity of the
environment on the refractive index of mesoporous layers in the
device;
[0040] FIG. 6 shows the reflection of an embodiment of the device
as a function of the wavelength at different degrees of air
humidity,
[0041] Table 1 shows embodiments of compositions by means of which
mesoporous layers can be formed;
[0042] Table 2 shows properties of the layer obtained by using the
embodiments 1-6 of Table 1;
[0043] Table 3 shows properties of the layer obtained by using the
embodiments 7-12 of Table 1;
[0044] Table 4 shows further compositions by means of which
mesoporous layers can be obtained, as well as the dielectric
constant and the porosity of the mesoporous layers;
[0045] Table 5 shows the hardness and the Young's modulus for
different compositions; and
[0046] Table 6 shows the sensitivity of mesoporous layers based on
different compositions to the degree of humidity.
[0047] FIG. 1 is a diagrammatic cross-sectional view of the
electronic device, which is not drawn to scale. The device shown in
this example is a semiconductor device 20. Said semiconductor
device 20 comprises a semiconductor substrate 1 provided with
conductors 3, 4, 5 at a surface 2. The conductors 3, 4, 5 each have
an upper surface 6 and side faces 7. It is noted that it is
possible that only one conductor is provided, although the
invention is described in the context of three conductors 3, 4, 5
and three vias 14, 15, 16. Customarily, however, the semiconductor
device comprises a large number of conductors and vias. Although
they are shown as one element, the semiconductor substrate 1
customarily comprises a plurality of layers formed on, for example,
a semiconductor body formed, for example, from silicon. The
conductors 3, 4, 5 can fulfill various functions. It is possible
that the conductors 3, 4, 5 are the gate electrodes of a
metal-oxide-semiconductor field effect transistor (MOSFET) or a
thin-film transistor (TFT). Alternatively, the conductors 3, 4, 5
can form the bases or emitters of a bipolar device or a BiCMOS
device. Furthermore, the conductors 3, 4, 5 may be part of a metal
layer of a multilayer interconnect structure.
[0048] The conductors 3, 4, 5 are composed of a metal portion 11
covered by a top layer 8 that serves as an anti-reflective coating.
In this example, the top layer 8 is a double layer of a layer of
titanium 9 and a layer of titanium nitride 10. The conductors 3, 4,
5 are formed in accordance with conventional process steps.
Subsequently, an etch stop layer 12 of silicon carbide is provided
at the upper surface 6 and the side faces 7 of the conductors 3, 4,
5 and also on the uncovered part of the surface 2 of the
semiconductor substrate 1.
[0049] The etch stop layer 12 is provided with a composition of
TEOS, ASAS, a surfactant and a solvent. Specific compositions are
listed in Table 1. For the solvent use is made, in this case, of a
mixture of alcohol, water and a small amount of acid. Suitable
alcohols include, inter alia, methanol, ethanol, propanol and
butanol. After drying and heating at 400.degree. C., the mesoporous
layer 13 is formed. It has been found that the thickness of the
layer formed depends on the number of revolutions during spin
coating, the viscosity of the composition and the degree of
dilution of the composition. If cetyltrimethylammoniumbromide
(CTAB) is used as the surfactant, the pore size is 2-3 nm; if
Pluronic F127 is used as the surfactant, the pore size is 7-8 nm.
Measurements using X-ray diffraction and SEM equipment show that
the pore size is substantially uniform.
[0050] The properties of this layer depend on the composition, as
listed in Table 2. Conductors 17, 18, 19, preferably of copper, are
present on the mesoporous layer 13. To preclude undesirable
diffusion of ions and particles, preferably, a barrier layer, not
shown, is applied to the mesoporous layer 13.
[0051] To pattern the mesoporous layer 13, a photoresist (not
shown) is provided. This photoresist is subsequently exposed in
accordance with a desired pattern and developed. As a result, a
photoresist mask is obtained having openings at the locations where
vias 14, 15, 16 are formed during filling with metal. The
mesoporous layer 13 is etched using a CVD treatment comprising, at
a pressure of 23.3 Pa (175 mTorr), 500 sccm Ar/50 sccm CF.sub.4 and
20 sccm CHF.sub.3. If the thickness of the mesoporous layer 13 over
the surface 2 of the semiconductor body 1 is not uniform, certain
vias can be subjected to a wet-chemical treatment for a
comparatively long period of time. To preclude reactions between
the etchants and the metal conductors 3, 4, 5, and in connection
with the occurrence of slightly misaligned vias, such as via 15,
the etch stop layer 12 is applied. This etch stop layer 12 is
removed at the location of the vias 14, 15, 16 to be formed by
means of, for example, a fluorocarbon in a dry, anisotropic etching
treatment. Subsequently, conductive material, such as aluminum,
copper or tungsten is provided and the vias 14, 15, 16 are formed.
Preferably, an adhesive layer and/or a barrier layer is deposited
prior to the deposition of the conductive material. Next, the
conductive material is polished by means of a conventional CMP
treatment.
EXAMPLE 1
[0052] A composition of tetraethoxyorthosilicate (TEOS),
methyltrimethoxysilane (MTMS), water and ethanol, which is
acidified with HCl, is formed while stirring. The molar ratios of
TEOS:MTMS:H.sub.2O:ethanol:HCl are 0.5:0.5:1:3:5.10.sup.-5. This
composition was heated to 60.degree. C. for 90 minutes. Water,
ethanol, HCl and cetyltrimethylammoniumbromide (CTAB) were added to
this pre-treated composition to obtain a molar ratio of
TEOS:MTMS:H.sub.2O:ethanol:HCl:CTAB of 0.5:0.5:7.5:20:0.006:0.10.
The composition was stirred for three days at room temperature.
Subsequently, the composition is provided by means of spin coating
at 1000 rpm for 1 minute in a KarlSuss CT62 spin coater. The layer
is dried at 130.degree. C. for 10 minutes on a hot plate and
subsequently heated to 400.degree. C. for 1 hour in air. In this
manner a mesoporous layer having a thickness of 200-400 nm is
obtained having a relative dielectric constant of 2.4 and a
porosity of 44%, as listed in Table 2.
[0053] In this case, the dielectric constant is measured by means
of a mercury probe (type Hg-612 from MSI electronics) at a
frequency of 1 MHz. The porosity is determined in at least one of
the two following ways known to persons skilled in the art: on the
basis of the refractive index and by means of a layer thickness
measurement and RBS. The refractive index is determined through
ellipsometry using a VASE ellipsometer VB-250, JA Woolam Co, Inc.
From this value the porosity is determined via a Bruggeman
effective medium approximation with a depolarization factor of
0.33.
EXAMPLE 2
[0054] A composition of TEOS, MTMS, water, ethanol, HCl and CTAB is
prepared, in which the amount of surfactant is increased, as
compared to example 1, to 0.22. The composition is treated in the
manner described in example 1. This leads to a layer having a
relative dielectric constant of 2.3 and a porosity of 56%.
EXAMPLE 3
[0055] The composition of example 2 is stirred for three days at
room temperature. Subsequently, the composition is provided by
means of spin coating at 1000 rpm for 1 minute in a KarlSuss CT62
spin coater. The layer is dried at 130 .degree. C. for 10 minutes
and subsequently heated to 400.degree. C. for 1 hour in a gas
mixture comprising 93 vol. % N.sub.2 and 7 vol. % H.sub.2. A layer
having a relative dielectric constant of 1.9 is obtained.
EXAMPLE 4
[0056] A composition of TEOS, MTMS, water, ethanol and surfactant
is prepared, wherein the quantity of MTMS is increased, as compared
to example 1, to TEOS:MTMS=0.4:0.6. In this case, for the
surfactant use is made of Brij76 (polyoxyethylene (10) stearyl
ether) in a concentration of 0.13 mol/mol siloxane. The composition
is treated in the manner described in example 1. This leads to a
mesoporous layer having a relative dielectric constant of 1.7 and a
porosity of 62.4%.
EXAMPLE5
Not in Accordance with the Invention
[0057] A composition is prepared of TEOS, water, ethanol, HCl and
CTAB in the ratio indicated in Table 1. The composition is stirred
at room temperature for three days. Subsequently the composition is
applied by means of spin coating at 1000 rpm for 1 minute in a
KarlSuss CT62 spincoater. The layer is dried at 130.degree. C. for
10 minutes and subsequently heated to 400.degree. C. in air for 1
hour. This leads to a mesoporous layer having a layer thickness of
200-400 nm and a relative dielectric constant above 6. The layer
contains moisture, which is corroborated in ellipsometric
measurements, the air humidity degree being varied.
1 no TEOS ASAS surfactant HCl H.sub.2O EtOH application heating 1
0.75 MTMS, CTAB, 0.004 5 20 dipping 1 hour at 400.degree. C. 0.25
0.08-0.14 in air 2 0.75 PhTES, CTAB, 0.004 5 20 Spin 1 hour at
350.degree. C. 0.25 0.1 coating in air 3 0.5 MTMS, CTAB 0.006 7.5
20 Spin 1 hour at 400.degree. C. 0.5 0.10-0.22 coating in air 4 0.5
MTMS, CTAB 0.006 7.5 20 Spin 1 hour at 400.degree. C. 0.5 0.10-0.22
coating in 7% H.sub.2 in N.sub.2 5* 1.0 0 CTAB 0.006 7.5 20 Spin 1
hour at 400.degree. C. 0.10-0.24 coating in air 6 0.5 MTMS, F127,
0.004 5 20 dipping 1 hour at 400.degree. C. 0.5 0.0052 in air 7 0.5
MTMS, 0.5 F127, 0.004 5 20 Spin 1 hour at 400.degree. C. 0.006
coating in air 8 0.5 MTMS, F127, 0.004 5 10 Spin 1 hour at
400.degree. C. 0.5 0.006 coating in air 9 0.5 MTMS, CTAB, 0.004 5
20 Spin 1 hour at 400.degree. C. 0.5 0.10 coating in air 10 0.5
MTMS, Brij 76, 0.004 5 20 Spin 1 hour at 400.degree. C. 0.5 0.14
coating in air 11 0.67 DMDES, CTAB, 0.004 5 20 Spin 1 hour at
400.degree. C. 0.33 0.18 coating in air 12 0.67 DMDES, CTAB, 0.004
5 20 Spin 1 hour at 400.degree. C. 0.33 0.18 coating in 7% H.sub.2,
in N.sub.2 *not in accordance with the invention
[0058] Table 1, compositions, way of applying and heating. The
figures listed indicate the molar ratios.
[0059] TEOS=tetraethoxyorthosilicate
[0060] CTAB=cetyltrimethylammoniumbromide
[0061] MTMS=methyltrimethoxysilane
[0062] PhTES=phenyltriethoxysilane
[0063] F127=Pluronic F127, a triblock polymer comprising
polyethylene oxide, polypropylene oxide and polyethylene oxide as
the blocks; Brij76=polyoxyethylene (10) stearyl ether,
C.sub.18H.sub.37(OCH.sub.2CH.s- ub.2)nOH, n.apprxeq.10
[0064] DMDES=dimethyldiethoxysilane
2 Surfactant no concentration porosity n.sub.i .epsilon..sub.r 1
0.08 45% 1.25 3.9 0.10 49% 1.23 3.1 0.12 54% 1.21 3.3 0.14 53% 1.21
3.3 2 0.1 45% 1.34 2.6 (<50% RH), 1.45 (>70% RH) 3 0.10 44%
1.25 2.4 0.13 50% 1.23 2.3 0.16 53% 1.21 2.2 0.19 53% 1.21 2.2 0.22
56% 1.20 2.3 4 0.10 45% 1.25 2.5 0.16 54% 1.20 2.0 0.22 56% 1.19
1.9 5* 0.10 46% 1.24 >6 0.13 47% 1.24 0.16 36% 1.29 0.19 29%
1.32 0.24 28% 1.33 6 F127/ 54% 1.20 1.8 0.0052
[0065] Table 2--porosity, refractive index n.sub.i and relative
dielectric constant .epsilon..sub.r of the mesoporous layers
prepared using the compositions 1-6 with varying quantities of
surfactant. Unless indicated otherwise, the surfactant used is
CTAB.
3 rpm during layer thickness no spin coating (nm) porosity n.sub.1
.epsilon..sub.r 7 1000 rpm 692 54% 1.20 750 rpm 851 57% 1.19 500
rpm 1030 57% 1.19 8 1000 rpm 1545 59% 1.20 750 rpm 1802 60% 1.19 9
1000 rpm 409 46% 1.24 750 rpm 473 46% 1.24 500 rpm 568 46% 1.24 10
1000 rpm 494 59% 1.18 1.8 11 1000 rpm 441 53% 1.21 2.6 12 1000 rpm
438 51% 1.22 2.5
[0066] Table 3 --layer thickness, porosity, refractive index
n.sub.i and relative dielectric constant .epsilon..sub.r of the
mesoporous layers prepared using the compositions 7-12 at a varying
number of revolutions during spin coating.
[0067] Table 4 shows compositions wherein the ASAS content is
higher than in the compositions listed in Table 1. The
abbreviations used are identical to those used in Table 1.
Mesoporous layers are prepared by applying the compositions to a
substrate by means of spin coating and subsequently heating these
compositions in air at 400.degree. C. for 1 hour. Table 4 also
shows the porosity and the relative dielectric constant
.epsilon..sub.r of the mesoporous layers.
4TABLE 4 no TEOS ASAS surfactant HCl H.sub.2O EtOH porosity
.epsilon..sub.r 13 0.4 MTMS, 0.6 CTAB, 0.10 0.004 5 20 45% 14 0.4
MTMS, 0.6 CTAB, 0.27 0.004 5 20 52% 1.8 15 0.4 MTMS, 0.6 Brij76,
0.004 5 20 60% 1.7 0.13-0.16 16 0.4 MTMS, 0.6 F127, 0.007 0.004 5
10 56% 1.75 17 0.25 MTMS, 0.75 CTAB, 0.1 0.004 5 20 42% 18 0.1
MTMS, 0.9 CTAB, 0.1 0.004 5 20 40%
[0068] FIG. 2 shows the porosity P of mesoporous layers as a
function of the surfactant concentration C. The concentration is
given in mol per mol of siloxane (total amount of TEOS and ASAS).
For the surfactant use is made of CTAB. The measurements indicated
by means of squares relate to a mesoporous layer in accordance with
the state of the art, which is obtained using a composition
comprising TEOS. The measurements indicated by means of diamonds
relate to a mesoporous layer in accordance with the invention,
which is obtained using a composition of TEOS and MTMS in a molar
ratio of 1:1. The measurements indicated by means of triangles
relate to a mesoporous layer in accordance with the invention,
which is obtained using a composition of TEOS and MTMS in a molar
ratio of 2:3.
[0069] At CTAB concentrations below 0.1, the porosity increases as
the concentration increases, and there is no difference between a
layer based on a composition of pure TEOS and a layer prepared by
means of the method in accordance with the invention. At a CTAB
concentration of 0.1 (mol/mol) the porosity is 40-45%. At CTAB
concentrations above 0.1 (mol/mol) the porosity of a mesoporous
layer based on pure TEOS no longer increases but instead decreases
to approximately 30%. If, however, compositions in accordance with
the invention are used, mesoporous layers having a higher porosity
up to 60% are obtained. At CTAB concentrations above 0.27 (mol/mol)
a slight decrease of the porosity to 45-50% is observed.
[0070] FIG. 3 shows the relative dielectric constant
.epsilon..sub.r as a function of the porosity P. The measurements
indicated by means of diamonds relate to a mesoporous layer in
accordance with the invention, which is obtained using a
composition of TEOS and MTMS in a molar ratio of 1: 1, wherein CTAB
is used as the surfactant. The measurements indicated by means of
circles relate to a mesoporous layer in accordance with the
invention, which is obtained using a composition of TEOS and MTMS
in a molar ratio of 2:3, wherein CTAB is used as the surfactant.
The measurements indicated by means of triangles relate to a
mesoporous layer in accordance with the invention, which is
obtained using a composition of TEOS and MTMS in a molar ratio of
2:3, wherein Brij76 is used as the surfactant. The line that
extends through the measurements carried out on layers based on
compositions comprising TEOS:MTMS=1:1 shows that a linear
relationship exists between dielectric constant and porosity. The
measurements carried out on layers based on compositions comrising
TEOS:MTMS=2:3 are situated slightly to the left of the line that
relates to TEOS:MTMS=1:1. This indicates that the same dielectric
constant is achieved already at a lower porosity.
[0071] Table 5 shows the porosity, the hardness and the Young's
modulus for a number of mesoporous layers. Said mesoporous layers
are prepared using the compositions listeded in Tables 1 and 4,
with the exception of layers 19 and 20. Said mesoporous layers are
known from S. Yang et.al., Chem. Mater. 14(2002), 369-374. Said
mesoporous layers are made from poly(methylsilsesquioxane) (MSQ),
wherein triblock polymers, i.e. poly(ethylene oxide-b-propylene
oxide-b-ethylene oxide), are used. These mesoporous layers are
prepared using a composition of MSQ precursors having an average
molecular weight M.sub.r,n of 1668 g/mol. The composition is a 30%
solution in n-butanol and further comprises said triblock polymer.
After filtration, the composition was applied to a substrate,
whereafter the liquid layer was dried at 120.degree. C. and heated
at 500.degree. C. It is noted that Yang et al. used a composition
with an MSQ precursor, which is a polymer already, as the starting
composition. In the invention, the starting composition comprises
TEOS and an ASAS, which are monomers.
5TABLE 5 hardness and Young's modulus for various mesoporous
layers. TEOS: hardness Young's no MTMS surfactant porosity (GPa)
modulus (GPa) 5* 1:0 CTAB, 0.1 49% 0.8 12-17 1 3:1 CTAB, 0.1 49%
0.7 8 3 1:1 CTAB, 0.1 45% 0.8 4.5 13 2:3 CTAB, 0.1 45% 0.8 5.4 14
2:3 CTAB, 0.27 52% 0.5 3.0 15 2:3 Brij76, 0.16 60% 0.5 3.2 16 2:3
F127, 0.007 56% 0.36 2.0 17 1:3 CTAB, 0.1 42% 0.6 3.5 18 1:9 CTAB,
0.1 40% 0.6 3.5 19* Not triblock F88 40% 0.28 1.3 applicable 20*
Not triblock F88 50% 0.16 0.6 applicable *= not in accordance with
the invention
[0072] The values listed in Table 5 show that the hardness of the
mesoporous layer in accordance with the invention decreases only
slightly as the TEOS:MTMS ratio decreases if use is made of a
constant type and concentration of the surfactant. Only the use of
higher concentrations of the surfactant CTAB or of a different
surfactant causes the porosity to increase and the hardness to
decrease. Said hardness levels and Young's moduli, however, are
still twice or thrice as high as the hardness values disclosed in
the publication by Yang et.al. Therefore, it can be concluded that
the mechanical strength of these layers is sufficient to withstand
chemical-mechanical polishing (CMP) during the manufacture of
integrated circuits.
[0073] Table 6 shows the porosity as a function of the air humidity
for mesoporous layers based on compositions having different molar
ratios of TEOS:MTMS. It can be concluded from the Table that by
using a composition comprising TEOS:MTMS<3:2, a mesoporous layer
is obtained which is hydrophobic also under conditions where the
air humidity is high. As regards the ratio TEOS:MTMS=3:1, it has
been found that humidification, and hence a reduction of the
porosity, takes place only at degrees of humidity above 80%.
Desorption of the adsorbed water is accompanied by a hysteresis
effect. During a subsequent increase of the degree of air humidity
adsorption already takes place from a degree of relative air
humidity of approximately 40%. If the degree of humidity does not
exceed 60%, humidification does not take place and the porosity
level remains high, resulting in a low refractive index and a low
dielectric constant.
6 composition low degree high degree (mol) of humidity of humidity
TEOS MTMS CTAB % RH porosity % RH porosity 1.0 0 0.10 2 45% 50* 12%
35** 0.75 0.25 0.10 2-70* 45% 85* 15% 0.6 0.4 0.15 1.6 54.3% 92
15.2% 0.55 0.45 0.15 2.6 51.9% 88.7 51% 0.50 0.50 0.15 2.5 53.2% 82
52.7% 0.40 0.60 0.15 2 51.1% 76 50.7% *at the first adsorption **at
desorption
[0074] Table 6 --the sensitivity to the degree of humidity of
mesoporous layers in accordance with the invention as a function of
the composition used to prepare the layer. The composition further
comprises the constituents listed in Table 1. The mesoporous layers
are prepared in accordance with example 1. %RH=relative degree of
humidity.
[0075] FIG. 4 shows the influence of the degree of air humidity on
the refractive index of various mesoporous layers prepared in
accordance with known methods. A change of the refractive index can
be attributed to water adsorption in the pores of the layer. This
is accompanied by an increase of the dielectric constant. Since the
diameter of the pores is small and the mesoporous layer is covered
by a subsequent layer in the device, water adsorption in a
mesoporous layer in a semiconductor device must be considered to be
irreversible in practice. The refractive index n.sup.550 is
measured in accordance with the above-mentioned method at a
wavelength of 550 nm.
[0076] The solid line shown in FIG. 4 relates to a mesoporous layer
of pure tetraethoxyorthosilicate. At a degree of humidity of 0%,
corresponding to anhydrous air, the refractive index is 1.22. At a
degree of humidity of 30%, the refractive index is 1.26 already,
and at 50%, the refractive index has increased to 1.40.
[0077] The dashed line in FIG. 4 relates to a mesoporous layer of
pure teatraethoxyorthosilicate that, after the provision of the
mesoporous layer, has been treated with trimethylchlorosilane
during drying. At a degree of humidity of 0%, the layer has a
refractive index of 1.27. At a degree of humidity of 60%, the
refractive index is 1.30, and at a degree of humidity of 80%, the
refractive index is 1.40. The relative dielectric constant is above
6 at degrees of humidity in excess of 30%.
[0078] In both cases the refractive index exhibits a hysteresis
effect. In the case of the TEOS layer that has not been subjected
to an aftertreatment, this hysteresis effect leads to a refractive
index of 1.38 at a degree of humidity of 35%. In the case of the
TEOS layer that has been subjected to an aftertreatment, hysteresis
is such that the refractive index is 1.40 at a degree of humidity
of 60% and 1.30 at a degree of humidity of 40%. The results
indicate that a substantial degree of water adsorption has taken
place under conditions occurring in industrial manufacturing
processes.
[0079] FIG. 5 shows the influence of the degree of air humidity on
the refractive index of various mesoporous layers forming part of
electronic devices in accordance with the invention.
[0080] The solid line (1) relates to a layer prepared from a
composition comprising tetraethoxyorthosilicate and
phenyltriethoxysilane in a molar ratio of 3:1. At a degree of
humidity of 0%, the refractive index is 1.33, and at an air
humidity of 50%, the refractive index is 1.335. At air humidity
levels of 60% and higher the refractive index increases, and at a
degree of humidity of 90% the refractive index is 1.45 If the
degree of humidity of 90% is reduced, a hysteresis effect occurs.
The relative dielectric constant is 2.6.
[0081] The dashed line (2) relates to a layer prepared from a
composition comprising tetraethoxyorthosilicate and
methyltrimethoxysilane in a molar ratio of 0.75:0.25. The
concentration of the surfactant CTAB is 0.10. At a degree of
humidity of 0%, the refractive index is 1.23, which value remains
the same at an air humidity level of 50%. At an air humidity level
of 70% and higher, the refractive index increases. If the degree of
humidity of 90% is reduced, a hysteresis effect occurs.
[0082] The dash-dot line (3) relates to a layer prepared from a
composition comprising tetraethoxyorthosilicate and
methyltrimethoxysilane in a molar ratio of 0.5:0.5. The
concentration of the surfactant CTAB is 0.10. The refractive index
of this layer is 1.25, independent of the air humidity level. The
relative dielectric constant is 2.4.
[0083] FIG. 6 relates to an embodiment of the device wherein a
substrate of silicon is provided with a stack of layers comprising
alternately a layer of TiO.sub.2 and a layer of porous
aryl-substituted or alkyl-substituted SiO.sub.2. Said stack of
layers comprises a total of several layers having a thickness as
indicated hereinbelow. The empirical formula of said
alkyl-substituted SiO.sub.2 is SiO.sub.1.875(Me).sub.0.12- 5. Said
alkyl-substituted SiO.sub.2 is manufactured using a composition
comprising TEOS and MTMS in a molar ratio of 3:1, wherein Pluronic
F127 is used as the surfactant.
7 layer no. material thickness (nm) n.sup.550 1 TiO2 53 2.245 2
SiOxMey 101 1.237 3 TiO2 65 2.172 4 SiOxMey 89 1.251 5 TiO2 65
2.152 6 SiOxMey 103 1.252 7 TiO2 65 2.116
[0084] In FIG. 6, the transmission T (in %) of the stack of layers
is indicated as a function of the wavelength .lambda. for two
different degrees of air humidity. The solid line relates to a
degree of air humidity of approximately 50% and is measured in air.
The dashed line relates to a degree of air humidity of less than 2%
and is measured in N.sub.2. The stack of layers can be used, for
example, as an interference stack, in which case the filter
characteristic can be controlled by means of air humidity or
temperature. The stack of layers can also be used for optical
storage of data, or for display screens and sensors. Inter alia by
varying the composition of the alkyl-substituted SiO.sub.2, the
high-low transmission transition can be set to a desired relative
air humidity or saturation vapor pressure between 10 and 90%. Said
transition can also be influenced by means of the pore size in the
layer. This pore size depends on the surfactant used. The degree to
which the transmission at a first degree of air humidity differs
from that at a second degree of air humidity depends on the
wavelength of the light coupled-in. This means that the change in
relative air humidity can be observed as a shift of the reflected
light. Such a stack can also be obtained using different mesoporous
layers, such as mesoporous TiO.sub.2 layers.
[0085] The above-mentioned porosities in the range from 40 to at
least 60%, the very low dielectric constant of 2.0 and less, and
the good mechanical stability causes the mesoporous layer that can
be obtained by means of the method in accordance with the invention
to be very suitable as an intermetallic or intrametallic dielectric
in a semiconductor device, particularly in an interconnect
structure of an integrated circuit. This also applies because a
suitable choice of ASAS enables thermal stability to temperatures
above 400.degree. C. to be obtained and because the mesoporous
layer has a dielectric constant that is comparatively or entirely
insensitive to the degree of air humidity of the atmosphere. In
addition, the pore size is uniform and below 10 nm, which precludes
diffusion of metal ions and other atoms, molecules or
particles.
* * * * *